1. Field of the Invention
The present invention relates to the manufacture of semiconductor devices and particularly fast switching semiconductor devices.
Certain semiconductor devices must be switched from a high current conducting mode to a high voltage blocking mode during their operation. The most common of these is thyristors where the switching time is measured as turn-off time (tq) and diodes where the switching time is measured as reverse recovery time (trr). These semiconductor devices also include transistors, discs, triacs, reverse switching rectifiers and reverse conducting thyristors.
It has recently been demonstrated to irradiate semiconductor devices to modify the electrical characteristics in various ways. See e.g. U.S. Pat. Nos. 3,809,582, 3,840,887, 3,852,612, 3,872,493, 3,877,997, 3,881,963, 3,881,964, 3,888,701 and 3,933,527, and U.S. Pat. applications Ser. Nos. 357,435 (filed May 4, 1973), 540,208 (filed Jan. 10, 1975), 581,255 (filed May 27, 1975) and 667,791 (filed Mar. 17, 1976), all of which have been assigned to the same assignee as the present invention.
More particularly, U.S. Pat. Nos. 3,881,963 and 3,809,582 teach to irradiate high power thyristors and diodes with radiation sources generally and electron radiation sources preferably to decrease the turn-off time and reverse recovery time of such devices. Such irradiation has been demonstrated to produce distinct advantages over gold diffusion previously employed to produce fast switching devices. However, these radiation techniques have been found to have their limitations where other electrical characteristics and particularly forward voltage drop are to be maintained. If a very low turn-off time in a thyristor or a very low reverse recovery time in a diode is desired, a higher forward voltage drop has to be tolerated in the device.
Therefore, simply irradiating thyristors or diodes to reduce the switching time has previously involved a trade-off to a greater or lesser degree with forward voltage drop. The limitation of this trade-off has been reduced to some degree by annealing processes described in U.S. Pat. No. 3,888,701, granted June 10, 1975. However, still some trade-off of switching time with forward voltage drop has been necessary.
More recently, U.S. Pat. application Ser. No. 667,791 has described a way of eliminating this need for annealing to relieve the trade-off involved in previous irradiation processes by irradiation with nuclear sources and preferably proton and alpha particle sources. However, this technique did not lend itself to mass production of semiconductor devices because of the limited availability of particle sources. Also, the devices still had to be oriented relative to the radiation source before irradiation. Commercial electron irradiation remained the best available technique for altering the switching time because it was more readily adapted to large scale production of devices.
The use of irradiation with neutrons has been generally suggested to applicants, but when tried the devices were found unacceptable. Semiconductor devices of high power capability, where irradiation has been useful, have usually been specified to maintain stable electrical characteristics at operating and storage temperatures of 200.degree. C. or more. The electrical characteristics of such devices irradiated with neutron radiation were found to be very sensitive to temperature changes and degrade at operating temperatures between 100.degree. and 200.degree. C.
The present invention overcame this difficulty. It provides a method of producing by simple irradiation semiconductor devices with superior electrical characteristics to those obtained by proton and alpha irradiation, yet with mass production capability comparable or greater than with electron irradiation. Additionally, it reduces time and expense in fabrication by electron irradiation because the devices need not be generally oriented on a flat surface preparatory to irradiation and can be fully encapsulated before they are irradiated.
A related problem for circuit designers in the application of thyristors and diodes is matching the recovery charge (Q.sub.rr) of the devices. For example, where diodes and thyristors must be used in series or in parallel to provide the desired high voltage or high current characteristics for the circuit, one of the most important parameters is the recovery charge of the individual devices. The recovery charge of each diode or thyristor must be matched with the other diodes or thyristors in the circuit so that each device supports substantially the same amounts of voltage or current within the specified limits. Otherwise, an individual device of the circuit having too high a recovery charge will support too much of the voltage or current and be burned out, causing the entire circuit to fail.
It has been demonstrated to match the recovery charge of diodes and thyristors by irradiating with electron radiation with specific radiation dosages. This technique is described and claimed in application Ser. No. 687,278, filed May 17, 1976 and assigned to the same assignee as this application. However, as in techniques using electron irradiation, a trade-off with other electrical characteristics and most notably forward voltage drop was involved, which limited applications of the technique. Further the devices still had to be generally oriented relative to the radiation source before irradiation. The present invention also overcame this difficulty.